linux-kernel - Re: [PATCH 1/4] sched/topology: SD_ASYM

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Date:   Mon, 23 Jul 2018 17:07:50 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Morten Rasmussen <morten.rasmussen@....com>
Cc:     vincent.guittot@...aro.org, peterz@...radead.org,
        linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        mingo@...hat.com, valentin.schneider@....com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/4] sched/topology: SD_ASYM_CPUCAPACITY flag detection

On 23/07/18 16:27, Morten Rasmussen wrote:

[...]

>>> +	/*
>>> +	 * Examine topology from all cpu's point of views to detect the lowest
>>> +	 * sched_domain_topology_level where a highest capacity cpu is visible
>>> +	 * to everyone.
>>> +	 */
>>> +	for_each_cpu(i, cpu_map) {
>>> +		unsigned long max_capacity = arch_scale_cpu_capacity(NULL, i);
>>> +		int tl_id = 0;
>>> +
>>> +		for_each_sd_topology(tl) {
>>> +			if (tl_id < asym_level)
>>> +				goto next_level;
>>> +
>> I think if you increment and then continue here you might save the extra
>> branch. I didn't look at any disassembly though to verify the generated
>> code.
>>
>> I wonder if we can introduce for_each_sd_topology_from(tl, starting_level)
>> so that you can start searching from a provided level - which will make this
>> skipping logic unnecessary? So the code will look like
>>
>>              for_each_sd_topology_from(tl, asymc_level) {
>>                  ...
>>              }
> Both options would work. Increment+contrinue instead of goto would be
> slightly less readable I think since we would still have the increment
> at the end of the loop, but easy to do. Introducing
> for_each_sd_topology_from() improve things too, but I wonder if it is
> worth it.

I don't mind the current form to be honest. I agree it's not worth it if 
it is called infrequent enough.

>>> @@ -1647,18 +1707,27 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
>>>   	struct s_data d;
>>>   	struct rq *rq = NULL;
>>>   	int i, ret = -ENOMEM;
>>> +	struct sched_domain_topology_level *tl_asym;
>>>   	alloc_state = __visit_domain_allocation_hell(&d, cpu_map);
>>>   	if (alloc_state != sa_rootdomain)
>>>   		goto error;
>>> +	tl_asym = asym_cpu_capacity_level(cpu_map);
>>> +
>> Or maybe this is not a hot path and we don't care that much about optimizing
>> the search since you call it unconditionally here even for systems that
>> don't care?
> It does increase the cost of things like hotplug slightly and
> repartitioning of root_domains a slightly but I don't see how we can
> avoid it if we want generic code to set this flag. If the costs are not
> acceptable I think the only option is to make the detection architecture
> specific.

I think hotplug is already expensive and this overhead would be small in 
comparison. But this could be called when frequency changes if I 
understood correctly - this is the one I wasn't sure how 'hot' it could 
be. I wouldn't expect frequency changes at a very high rate because it's 
relatively expensive too..

> In any case, AFAIK rebuilding the sched_domain hierarchy shouldn't be a
> normal and common thing to do. If checking for the flag is not
> acceptable on SMP-only architectures, I can move it under arch/arm[,64]
> although it is not as clean.
>

I like the approach and I think it's nice and clean. If it actually 
appears in some profiles I think we have room to optimize it.

-- 
Qais Yousef